To make a moulds from the original master copy of a
sculpture which can then be used to accurately reproduce
replicas of an original sculpture is usually undertaken at the art foundry,
however briefly outlined here is an example of how a one and two
piece mould was made which would enable wax replicas of the
original sculptures to be reproduced.

Silicon rubbers which was used in these two examples
can be effective for a variety of mould making requirements and by
using a variety of combinations of silicon rubbers, catalysts and additives enable
a wide range of strengths, viscosities and properties to be obtained. Room
vulcanising i.e. cold cure rubbers can produce moulds with working temperatures
of between -50° C to + 250° C to cast various materials including wax,
cement, plaster, polyesters and epoxies. The silicon rubber used in
this example was a cold cure, very high strength grey rubber and proved very effective
in replicating all the fine detail required..

As way of an example two completely different style of
sculpture were to be the subject, although they had a number of
undercuts which can sometimes prove to be problematic, it was still
possible to make a rubber and plaster mould without damage to the original
master copy and also the wax replicas themselves once they were
cast and removed from the mould.

The main materials required to produce this particular mould
was a high strength moulding plaster, which is required to retain the
flexible silicon rubber mould itself in the correct position. The two part cold
cure silicon rubber, when necessary some 'Thixo' agent which thickens the
rubber when added to the rubber mixture, a small box with removable sides to
contain the plaster as it was being poured. Scales or some other suitable means
to accurately measure the correct mixture of the two parts of silicon
rubber. A supply of soft clay which was used amongst other things as a
'bed' for the sculpture master copy to be placed upon, and 'soft
moulding soap' or a 'spray on mould release agent', which helped prevent
the two separate sections of plaster and silicon rubber sticking to each other and
also the master copy itself when the various casting materials i.e. wax,
plaster etc, were being poured.

The original sea
otter and bald eagle sculptures use to produce silicon rubber
moulds

One piece Silicon rubber
mould :-

The original sea otter sculpture produced by Ian was
used in this example in making a one piece mould. Thin shims of waxed cardboard
were used to keep the original sculpture slightly raised
above the surface of the base to enabled the silicon rubber to flow
underneath the wax original a few centimeters.

Before the catalyst which cures the silicon rubber was
added, the rubber was firstly mixed with a Thixo type additive
which thicken the rubber, when the catalyst was then been
added to the mixture it can be applied to the sculpture with a brush
or spatula .Once the whole sculpture has been covered with this rubber mix to
the desired thickness on this occasion to a depth of around 3 mm, it is
left overnight to cure.

The now cured silicon rubber covered sculpture was then
surrounded with a sheet of cardboard embedded in a soft clay which helps to contain
the moulding plaster which could then be slowly poured over the rubber
mould.

Once the plaster has virtually set the cardboard wall
was carefully removed and the still slightly warm moulding plaster trimmed to
the desired shape. The plaster once it has set hard was turned over and the
waxed cardboard shims removed, the original sea otter sculpture could then be
taken from the rubber mould.

The rubber mould was then held up to the light to check for
any holes or potentially thin sections in the rubber, any such
imperfections can be usually be corrected at this stage by making up a
small mixture of the same silicon rubber and thixo mixture and then add it
to any thin or damaged sections as this silicon rubber mixture will
usually successfully stick to each other to form an adequate seal.

The silicon rubber mould was then cleaned with warm soapy
water and allowed to dry. To produce the first wax castings hot wax
was carefully dripped into the rubber mould and swilled around the
mould until the desired thickness was obtained, this wax would eventually be
replaced using the loss wax process by bronze.

Creating a two-piece silicon rubber
mould

To make a multi piece mould from the original master copy of a
sculpture without damaging the master copy, which can then be
used to successfully reproduce many accurate replicas of the original
sculpture in a variety of materials is a highly skilled process
usually undertaken at the art foundry. However briefly outlined here is
one example of how a simple two piece mould was produced and
then used to cast wax replicas.

Silicon rubbers which has been used in this example can be effective
for most mould making requirements. By using a variety of combinations
of silicon rubbers, catalysts and additives which enables a
wide range of strengths, viscosities and properties. Room vulcanising i.e.
cold cure rubbers can produce moulds with working temperatures of between
-50° C to + 250° C to cast various casting materials, which include
wax, cement, plaster, polyesters and epoxies. The silicon rubber used
in this example was a cold cure very high strength grey rubber and proved very effective
in replicating all the fine detail.

On this occasion one of Ian's smaller sculptures, a bald eagle was to
be the subject, although the sculpture has a number of undercuts which
can sometimes prove to be problematic, it was fortunately however
possible to make a mould without damage to the original master copy and the wax
replicas themselves when they were cast, also they did not requiring
to much detailed work to finish them after they were removed from
the mould.

The main materials required to produce this particular mould was a
high strength moulding plaster, which is required to retain the
flexible silicon rubber in position. The two part cold cure silicon rubber.
A small box with removable sides to contain the plaster as it
was being poured. Scales or a suitable measurement container which will
accurately measure the correct mixture of the two parts of silicon rubber.
A supply of soft clay which was used amongst other things as a 'bed'
for the sculpture master copy to be placed upon, and 'soft
moulding soap' or a 'spray on mould release agent', which helped
prevent the two separate sections of plaster and silicon rubber sticking to
each other and the master copy when the various materials were
being poured.

The
original 12 inches high woodcarving

The master copy of the Bald eagle was originally carved from lime
wood and then the sculpture was bleached and stained before
the surface of the wood was sealed with a wood sealant to help prevent
the rubber mould making material sticking to the all fine detail.

The making of this 'negative'
within the mould began once the most suitable joint-lines on the
original sculpture has been decided upon, this would
enabled the sculpture to be removed from the mould, without minimal
damaging to the original woodcarving, the rubber part of the
mould or indeed the wax replica themselves, once cast and being removed
from within the mould.

A very thin coat casting soap was applied to the whole surface
of the original wood sculpture which will help prevent the rubber
sticking to the woodcarving, sometimes a thin coat of wax can also be
used. A thin plastic sheeting was used to cover over the
whole sculpture which helped prevent the soft clay when it was
laid on to the sculpture clogging the fine detail on the master copy
which has to be cleaned of before the rubber can be poured..

A thin layer of soft clay was rolled out over the whole sculpture,
this layer of clay will determine the eventual thickness of the
rubber as this clay will eventually be replaced by the silicon rubber.
Small clay 'keys' are made and placed around the edge of the sculpture
to enable the rubber mould to be securely seated within its
plaster jacket. Raised sections made from rolled clay are
placed upon the sculpture not only to act as funnels and breather
holes which will be used not only to pour the liquid rubber into, but
will also help to secure the rubber in position within its plaster
cast 'jacket.'

A mixture of moulding plaster was then prepared and slowly poured
over the whole sculpture to minimise air bubbles until it was completely covered to
make the 'mother-mould'. Once the plaster has set usually within 30
minutes the whole box was then turned over and one of the sides of
the box is removed . Two shallow holes were placed into the inside
of the plaster jacket on either side of the mould, which enabled
the plaster jacket to be accurately match when they were placed
together.

The thickness of the clay,
later to be replaced by silicon rubber can be seen here

The clay that once formed the bed for the master copy to lay upon,
can now be removed and the back of the eagle sculpture is again covered with
a thin layer of clay to the desired thickness along with the clay
keys and poring funnels. Two small indentations to act as 'keys' were made
within the clay bed which will later be replaced by the
plaster, this will enable the two plaster jackets to key together
in the same position each time they are assembled.

The
plaster now almost fully covering the sculpture. The clay
having been removed from the left side of the plaster jacket.

The
side of the boxpreviously
removed is now screwed back into position and once again the
mixture of moulding plaster is slowly poured into the box, to
again cover the sculpture. Once the plaster has set the side of the box was
again removed and the two plaster mother moulds are then
carefully separated. The clay from one half of the two plaster
jackets was extracted to leave the negative or 'cavern'
which the liquid rubber when it is being poured will eventually flow
into.

The end
of a paint brush was used to make small indentations a
little way into the soft clay all around the side of the sculpture,
which will form a key to enable each separate halve of
the inner silicon rubber mould to be fitted accurately together
within the plaster jacket.

The silicon rubber
after being poured into the mould

The sculpture was then carefully
cleaned of all debris that might have been left on both the
master copy or within the plaster cavity itself, any debris not
removed will obviously spoilt the detail of the mould after the
rubber was poured over the sculpture. The inside of the
plaster jacket was brushed firstly with shellac and once dry
covered over with a moulding soap. The master copy itself
was then sprayed with a mould release agent which helped
to prevent the rubber when poured from sticking to either the
sculpture or plaster jacket.

Thetop 'hollowed' out section of
the plaster jacket was then carefully lowered over the sculpture
which is still firmly imbedded into the clay bed. The clay closely
follows along the edges of the master copy (where the eventual join line
of the two piece rubber mould will be). Once the top plaster jacket is
lowered over the sculpture all the seams of the plaster jacket were
sealed with soft clay to help prevent seepage through this join when the
rubber is being poured into the mould. Simple clay funnels are also
placed upon the top of the mould which the liquid rubber is poured
into.

The two part silicon rubber and catalyst is
carefully measured and then mixed together before being slowly poured
into the space left by the clay, this mixture is then allowed to
cure overnight. At this early stage Ian sometimes allows all
the overspill of rubber on the top of the plaster mould to remain in
position which will often help keep this part of the mould and
sculpture securely in place once the mould has been turned over
to repeat the operation on the other half of the mould.

The recently cured grey rubber section
of the mould can be seen on the right hand part of the
plaster mould, along with the 'pencil' holes for aliening the two
parts of the silicon rubbers together. The plaster jacket above on the
left, once assembled together is now ready to be filled with
the silicon rubber. It is important that the half of the mould
containing the cured rubber and the master copy itself is not
disturbed in any way at this stage as when the new batch of
rubber is poured. Any gaps made down the side of the sculpture and plaster
jacket will quickly be filled when the new mixture of rubber
is poured which can sometimes cause problems when you try and
separate the mould.

Again the two halves of the plaster mould
are carefully placed together and the simple clay pouring holes for the
silicon rubber are again placed on the top and all the seams along
the edge of the plaster jacket are again sealed with soft
clay. The sculpture and the inside of the plaster jacket are once
again spray with a mould release agent.

A new
batch of silicon rubber was measured and the liquid rubber was again
poured into the plaster casting into the rubber section of the mould. Once
the master copy is totally enveloped with the moulding rubber and
has cured overnight, the plaster 'jacket' was removed to expose the
completed silicon grey rubber mould.

When mould making once the mould had been opened, on occasions gaps
may be found where perhaps the poured rubber has not reached and
covered all the original master copy. Providing the surface of the
rubber remains clean and the master copy has not been moved from its
original position, it is often possible to mix a further patch
of rubber and carefully cover over these gaps, as this new mixture of silicon
rubber will usually firmly stick to the previously silicon rubber.

Care still has to be taken when the master copy is removed
from the now cured rubber, as the master copy although
covered with the release agent and the woodcarving has
a sealant applied to the wood, the rubber still often will try
and stick to the still rather porous wooden master copy. The rubber
mould was then cleaned with soapy water and once dry, the rubber was
reassembled together and placed within its plaster jacket. The
edges of the mould was again sealed with a soft clay, as an extra
safety measure help prevent seepages from molten wax when it was
eventually poured into the mould.

The
completed two piece rubber mould and the firstwax
sample casting.

If the original sculpture tends to have a lot of undercuts which
are then to be found in the mould, it sometimes helps to remove
the wax replica's when they are still slightly warm as he wax is a
little more flexible. The wax casting of the eagle is then ready to
be placed upon a 'wax tree' and prepared to be for cast in bronze.

Tosee
the various stages of the process of casting a bronze, please click
this link toCreating
a Bronze.